Insights into the Understanding of the Nickel-Based Pre-Catalyst Effect on Urea Oxidation Reaction Activity

Nickel-based catalysts are regarded as the most excellent urea oxidation reaction (UOR) catalysts in alkaline media. Whatever kind of nickel-based catalysts is utilized to catalyze UOR, it is widely believed that the in situ-formed Ni3+ moieties are the true active sites and the as-utilized nickel-based catalysts just serve as pre-catalysts. Digging the pre-catalyst effect on the activity of Ni3+ moieties helps to better design nickel-based catalysts. Herein, five different anions of OH−, CO32−, SiO32−, MoO42−, and WO42− were used to bond with Ni2+ to fabricate the pre-catalysts β-Ni(OH)2, Ni-CO3, Ni-SiO3, Ni-MoO4, and Ni-WO4. It is found that the true active sites of the five as-fabricated catalysts are the same in situ-formed Ni3+ moieties and the five as-fabricated catalysts demonstrate different UOR activity. Although the as-synthesized five catalysts just serve as the pre-catalysts, they determine the quantity of active sites and activity per active site, thus determining the catalytic activity of the catalysts. Among the five catalysts, the amorphous nickel tungstate exhibits the most superior activity per active site and can catalyze UOR to reach 158.10 mA·cm–2 at 1.6 V, exceeding the majority of catalysts. This work makes for a deeper understanding of the pre-catalyst effect on UOR activity and helps to better design nickel-based UOR catalysts.


Introduction
Substituting renewable energies for fossil fuels is an effective method to conquer energy and environment problems at the same time, which is an urgent need for our society [1][2][3][4][5][6].However, renewable energies like solar and wind have drawbacks such as intermittency and transportation difficulties.Water electrolysis powered by renewable energies can transform these renewable energies into hydrogen molecules which can solve the drawbacks of renewable energies [7][8][9][10][11][12].Water electrolysis is composed of the hydrogen evolution reaction (HER) at the anode and oxygen evolution reaction (OER) at the cathode [13][14][15].Compared with HER, OER makes up the majority of energy waste in water electrolysis due to the inherent sluggish kinetics [16][17][18][19][20]. Changing the OER by the urea oxidation reaction (UOR) can extremely decrease the overpotential of overall water electrolysis, thus improving the utilization of renewable energies [1,3,10,19,20].Therefore, the design of cheap and efficient UOR catalysts has received much attention from scientists.
Since the discovery of urea electrolysis, platinum and rhodium have been regarded as the most superior UOR catalysts [21].The rare reserves and high cost of platinum and rhodium prompt the scientists to design new catalysts.Nickel-based catalysts bio-inspired from urease have received much attention since their discovery and they exhibit comparable UOR performance to precious metals platinum and rhodium [7,[22][23][24][25].In order to obtain more active nickel-based catalysts, various types of nickel-based catalysts were developed to catalyze UOR mainly spanning from nickel oxides [17,24,[26][27][28][29][30], nickel X-ides (X refers to C, N, P, S, Se, Te, etc.) [3,11,22,25,31], nickel-based composites [9,23,32], etc.For nickel oxide-related catalysts, Chen's group added Rh nanoparticles to NiO nanosheets which outperformed the counter NiO nanosheets [27].Rh and NiO are both UOR active components, and the addition of Rh nanoparticles endows the NiO nanosheet with a superior electronic structure.For nickel X-ide-related catalysts, Zhang's group supported nickel nitride nanospheres on nickel foam, which can efficiently catalyze the hydrogen generation and urea decomposition [31].The superior electron conductivity of the support nickel foam brought out the catalytic performance of nickel nitride.For nickel-based compositerelated catalysts, Hameed's group developed a type of NiO/graphene composite by a facile coprecipitation and calcination method [33].The introduction of graphene improves the conductivity of NiO and alters the electronic structure of NiO, and the NiO/graphene composite demonstrates more enhanced UOR activity than NiO.Through lots of in situ experimental research by advanced instruments and many theoretical investigations, although various types of nickel-based UOR catalysts are synthesized and demonstrate different UOR performances, it is widely believed that the as-synthesized catalysts just serve as the pre-catalysts and the in situ-formed Ni 3+ moieties are the true active sites [24,25,34].Since the true active sites are all Ni 3+ moieties, we question why different nickel-based catalysts demonstrate different UOR activity and what the significance of synthesizing different types of nickel-based catalysts is Systematic investigation about the effect of precatalysts on UOR activity helps to understand the meaning of synthesizing different kinds of catalysts and will lay a great foundation for further enhancing the UOR performance of nickel-based catalysts.

Determining Factors of UOR Activity
Catalytic activity is the sum of activity per active site.Therefore, the activity per active site and the quantity of active sites co-determine the catalyst activity [49].Charge transfer resistance reflecting the UOR barrier was evaluated by EIS [38].The UOR EIS diameter follows this sequence:

Determining Factors of UOR Activity
Catalytic activity is the sum of activity per active site.Therefore, the activity per active site and the quantity of active sites co-determine the catalyst activity [49].Charge transfer resistance reflecting the UOR barrier was evaluated by EIS [38]   The quantity of active sites: It is widely believed that Ni 3+ moieties are the true active sites and the pre-catalysts just serve as the precursors during UOR catalysis [11,25,31,50].It is crucial to untangle the relationship among the Ni 2+ molar mass, the Ni 3+ molar mass, the reduction equivalent (the capacity to accumulate Ni 3+ ) and UOR activity.The total Ni 2+ molar mass (calculated from the catalyst loading on glass carbon electrode) follows this sequence: 4b).The Ni 3+ molar mass (calculated from the reduction peak in Figure S4b (Figure 5e).The summary of the total Ni 2+ molar mass, Ni 3+ molar mass, and UOR activity does not exhibit a reasonable logic relationship, suggesting that the activity per active site cannot solely determine the UOR activity as well (Figure 5f).5e).The summary of the total Ni 2+ molar mass, Ni 3+ molar mass, and UOR activity does not exhibit a reasonable logic relationship, suggesting that the activity per active site cannot solely determine the UOR activity as well (Figure 5f).

Effect of the Pre-Catalyst Catalyst on UOR Activity
To verify if the true active sites are Ni 3+ moieties for Ni-WO4 and Ni-CO3, TEM and XPS were adopted to examine Ni-WO4 durability and Ni-CO3 durability.Catalyst nanoparticles and Ketjen black are observed in TEM images of Ni-WO4 durability and Ni-CO3 durability (Figure 6a,c).HRTEM images of Ni-WO4 durability and Ni-CO3 durability both demonstrate a facet with a width of 2.08 Å ascribed to the (200) facet of nickel (oxy)hydroxide (Figure 6b,d) [22,35].Ni 2p3/2 XPS of Ni-WO4 durability and Ni-CO3 durability are fitted with Ni 2+ (856.10 eV) and Ni 3+ (857.80 eV), followed by a satellite (862.70 eV) [36], confirming the formation of Ni 3+ moieties.W 4f XPS of Ni-WO4 possesses three peaks of 4f7/2 (34.98 eV), 4f5/2 (37.16 eV), and tungsten oxide loss feature (40.81 eV) (Figure 6f) [53,54].W signal cannot be detected in the corresponding binding energy range for Ni-WO4 durability, suggesting that element W on the Ni-WO4 surface was completely dis-

Structure Characterization
An X-ray diffractometer (XRD, DX-2700BH, Dandong Haoyuan instrument, Dandong, China) was used to collect XRD data with Cu K α radiation at a scan rate of 2 • •min −1 .A transmission electron microscope (TEM, JEOL JEM-2100F, Japan Electronics Co., Ltd, Showima City, Tokyo, Japan) was used to observe catalysts' morphologies with an accelerating voltage of 200 kV.An X-ray photoelectron spectroscope (XPS, Axis Supra, Shimadzu, Japan) was used to analyze element chemical states.A Shirley background was applied to all the XPS spectra and binding energies of all XPS spectra were charge referenced to 284.8 eV.The equation N(ε)εdε/ N(ε)dε was applied to calculate the d band center, where N(ε) represents the density of states.

Electrochemcial Analysis
The UOR performance was evaluated in 1 M KOH with 0.33 M urea.The as-synthesized catalysts, a Hg/HgO electrode and a carbon rod, were used as the working, reference, and counter electrodes.Working electrodes were prepared by casting 30 µL catalyst inks onto a glass carbon electrode polished by a polishing cloth using 50 nm Al 2 O 3 .Catalysts (4 mg) and ketjen black (0.5 mg) dispersed into the mixed solution of ethanol (1000 µL) and Nafion (40 µL) were used as the catalyst ink.Cyclic voltammetry (CV) curves scanned at 5 mV•s -1 , chronopotentiometry (CP) curves measured at 10 mA•cm -1 , electrochemical impedance spectroscopy (EIS) tested at 1.4 V from 100 mHz to 100 kHz, and the rotating ring-disk electrode (RRDE) method were used to analyze the UOR performance.The electrochemical data were collected after 10 cycles of CV curves in 1 M KOH.All the potentials were calibrated by the equation E RHE = E Hg/HgO + 0.098 + 0.059 × pH.The reduction equivalent was calculated by the equation RE = n(Ni 3+ )/n(Ni 2+ ).

Conclusions
In summary, β-Ni(OH) 2 , Ni-CO 3 , Ni-SiO 3 , Ni-MoO 4 , and Ni-WO 4 were fabricated by bonding Ni 2+ with OH − , CO 3 2− , SiO 3 2− , MoO 4 2− , and WO 4 2− anions, respectively.Through systematic investigations about the catalysts structure and UOR mechanism, the five as-fabricated catalysts are found to serve as pre-catalysts and the true active sites are the in situ-formed Ni 3+ moieties.The pre-catalyst has effects on the quantity of active sites and the activity per active site to determine the UOR activity.The as-fabricated amorphous nickel tungstate possesses the most superior activity per active site and a relatively high number of active sites, thus demonstrating excellent UOR activity to catalyze the UOR reaching 158.10 mA•cm -2 at 1.6 V, exceeding the majority of the reported catalysts.This work provides a deeper understanding of the pre-catalyst effect on UOR activity.

Figure 4 .
Figure 4. (a) UOR EIS, (b) total Ni 2+ molar mass, (c) Ni 3+ molar mass transformed from Ni 2+ , (d) reduction equivalent, and (e) summary of UOR performance of β-Ni(OH)2, Ni-CO3, Ni-SiO3, Ni-MoO4, and Ni-WO4.The activity per active site: The activity per active site is determined by the electronic structure of catalysts.The electronic structure was analyzed by valence band (VB) spectra shown in Figure 5a.VB spectra of β-Ni(OH)2, Ni-CO3, and Ni-SiO3 demonstrate similar shapes and are just located at slightly different binding energies.VB spectra of Ni-MoO4 and Ni-WO4 exhibit similar shapes but significantly differ from those of β-Ni(OH)2, Ni-CO3, and Ni-SiO3 especially below 6 eV.This indicates that Ni-MoO4 and Ni-WO4 possess significantly different electronic structures from β-Ni(OH)2, Ni-CO3, and Ni-SiO3 [7].This phenomenon is due to the fact that transition metals mainly contribute below 6 eV in VB spectra [51].D band centers were calculated from VB spectra (Figure 5b).D band center positions follow this sequence: Ni-CO3 (−4.78 eV) > β-Ni(OH)2 (−4.81 eV) > Ni-SiO3 (−4.89 eV) > Ni-MoO4 (−5.46 eV) > Ni-WO4 (−5.57eV).The lower the d band center position, the weaker the bonding strength between the catalyst and intermediates.The bonding strength between the catalyst and intermediates subject to d band center position [51,52] and the bonding scheme are depicted in Figure 5c.The desorption of the UOR intermediates is the rate-controlling step and Ni-WO4 demonstrates the lowest d band center, which facilitates the desorption of the UOR intermediates.Therefore, Ni-WO4 exhibits the highest activity per active site.For the direct analysis of the activity per active site, the UOR
) follows this sequence: β-Ni(OH) 2 > Ni-CO 3 > Ni-MoO 4 > Ni-WO 4 > Ni-SiO 3 (Figure 4c).The reduction equivalent follows this sequence: Ni-MoO 4 > β-Ni(OH) 2 > Ni-CO 3 > Ni-WO 4 > Ni-SiO 3 (Figure 4d).The summary of the relationship among the Ni 2+ molar mass, the Ni 3+ molar mass, reduction equivalent and UOR activity is shown in Figure 4e.The Ni 2+ molar mass, Ni 3+ molar mass, reduction equivalent and UOR activity do not exhibit a reasonable logic relationship, indicating that the quantity of active sites cannot solely decide the UOR activity.The activity per active site: The activity per active site is determined by the electronic structure of catalysts.The electronic structure was analyzed by valence band (VB) spectra shown in Figure 5a.VB spectra of β-Ni(OH) 2 , Ni-CO 3 , and Ni-SiO 3 demonstrate similar shapes and are just located at slightly different binding energies.VB spectra of Ni-MoO 4 and Ni-WO 4 exhibit similar shapes but significantly differ from those of β-Ni(OH) 2 , Ni-CO 3 , and Ni-SiO 3 especially below 6 eV.This indicates that Ni-MoO 4 and Ni-WO 4 possess significantly different electronic structures from β-Ni(OH) 2 , Ni-CO 3 , and Ni-SiO 3 [7].This phenomenon is due to the fact that transition metals mainly contribute below 6 eV in VB spectra [51].D band centers were calculated from VB spectra (Figure 5b).D band center positions follow this sequence: Ni-CO 3 (−4.78eV) > β-Ni(OH) 2 (−4.81 eV) > Ni-SiO 3 (−4.89eV) > Ni-MoO 4 (−5.46 eV) > Ni-WO 4 (−5.57eV).The lower the d band center position, the weaker the bonding strength between the catalyst and intermediates.The bonding strength between the catalyst and intermediates subject to d band center position [51,52] and the bonding scheme are depicted in Figure 5c.The desorption of the UOR intermediates is the rate-controlling step and Ni-WO 4 demonstrates the lowest d band center, which facilitates the desorption of the UOR intermediates.Therefore, Ni-WO 4 exhibits the highest activity per active site.For the direct analysis of the activity per active site, the UOR CV curves were normalized by the Ni 2+ and Ni 3+ molar mass.The UOR activity per Ni 2+ molar mass follows this sequence: β-Ni(OH) 2 ≈ Ni-CO 3 < Ni-SiO 3 < Ni-MoO 4 < Ni-WO 4 (Figure 5d).The UOR activity per Ni 3+ molar mass follows this sequence: Ni-CO 3 < Ni-SiO 3 < β-Ni(OH) 2 < Ni-MoO 4 < Ni-WO 4

Figure 5 .
Figure 5. (a) Valence band spectra, (b) d band models derived from valence band spectra, (c) bond formation scheme, (d) UOR CV curves normalized by the total Ni 2+ molar mass, (e) UOR CV curves normalized by Ni 3+ molar mass, and (f) summary of UOR performance of β-Ni(OH)2, Ni-CO3, Ni-SiO3, Ni-MoO4, and Ni-WO4.The activity per active site and the quantity of active sites co-determines the UOR activity of β-Ni(OH)2, Ni-CO3, Ni-SiO3, Ni-MoO4, and Ni-WO4.Ni-WO4 with the lowest d band center position demonstrates the highest activity per active site and possesses slightly fewer active sites compared to Ni-CO3 with the maximum quantity of active sites.Therefore, Ni-WO4 exhibits the highest UOR activity.

Figure 5 .
Figure 5. (a) Valence band spectra, (b) d band models derived from valence band spectra, (c) bond formation scheme, (d) UOR CV curves normalized by the total Ni 2+ molar mass, (e) UOR CV curves normalized by Ni 3+ molar mass, and (f) summary of UOR performance of β-Ni(OH) 2 , Ni-CO 3 , Ni-SiO 3 , Ni-MoO 4 , and Ni-WO 4 .The activity per active site and the quantity of active sites co-determines the UOR activity of β-Ni(OH) 2 , Ni-CO 3 , Ni-SiO 3 , Ni-MoO 4 , and Ni-WO 4 .Ni-WO 4 with the lowest d band center position demonstrates the highest activity per active site and possesses slightly fewer active sites compared to Ni-CO 3 with the maximum quantity of active sites.Therefore, Ni-WO 4 exhibits the highest UOR activity.2.4.Effect of the Pre-Catalyst Catalyst on UOR Activity To verify if the true active sites are Ni 3+ moieties for Ni-WO 4 and Ni-CO 3 , TEM and XPS were adopted to examine Ni-WO 4 durability and Ni-CO 3 durability.Catalyst nanoparticles and Ketjen black are observed in TEM images of Ni-WO 4 durability and Ni-CO 3 durability (Figure 6a,c).HRTEM images of Ni-WO 4 durability and Ni-CO 3 durability both demonstrate a facet with a width of 2.08 Å ascribed to the (200) facet of nickel (oxy)hydroxide (Figure 6b,d) [22,35].Ni 2p 3/2 XPS of Ni-WO 4 durability and Ni-CO 3